U.S. patent application number 13/903026 was filed with the patent office on 2013-11-28 for electrochemical sensor apparatus and electrochemical sensing method.
The applicant listed for this patent is Process Instruments (UK) Limited. Invention is credited to Jonathan Frank Cook, Peter Robert Fielden, Nicholas John Goddard, Stefanie Moorcroft, Michael Laurence Riding, Craig Stracey.
Application Number | 20130313128 13/903026 |
Document ID | / |
Family ID | 46545996 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130313128 |
Kind Code |
A1 |
Fielden; Peter Robert ; et
al. |
November 28, 2013 |
Electrochemical Sensor Apparatus and Electrochemical Sensing
Method
Abstract
An electrochemical sensor apparatus and electrochemical sensing
method within an aqueous system are described, using one or more
working electrodes of boron doped diamond (BDD). A cathodic
reduction process provides a cathodic measurement and,
substantially simultaneously, an anodic oxidation process provides
an anodic measurement. A sum of a content of two equilibrium
species within the aqueous system is obtained using both the
cathodic measurement and the anodic measurement. One example
measures total free chlorine by simultaneously measuring
hypochlorous acid (HOCl) and hypochlorite ion (OCl.sup.-).
Inventors: |
Fielden; Peter Robert;
(Rossendale, GB) ; Goddard; Nicholas John;
(Manchester, GB) ; Moorcroft; Stefanie; (Salford,
GB) ; Stracey; Craig; (Burnley, GB) ; Cook;
Jonathan Frank; (Oldham, GB) ; Riding; Michael
Laurence; (Burnley, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Process Instruments (UK) Limited |
Lancashire |
|
GB |
|
|
Family ID: |
46545996 |
Appl. No.: |
13/903026 |
Filed: |
May 28, 2013 |
Current U.S.
Class: |
205/780 ;
204/406; 205/793.5 |
Current CPC
Class: |
G01N 27/308 20130101;
G01N 27/4168 20130101; G01N 33/182 20130101 |
Class at
Publication: |
205/780 ;
205/793.5; 204/406 |
International
Class: |
G01N 27/416 20060101
G01N027/416 |
Foreign Application Data
Date |
Code |
Application Number |
May 28, 2012 |
GB |
1209359.7 |
Claims
1. An electrochemical sensing method suitable for measuring an
aqueous system, the method comprising: measuring a cathodic
reduction process using a working electrode of boron doped diamond
to provide a cathodic measurement; measuring an anodic oxidation
process using a boron doped diamond working electrode to provide an
anodic measurement; and outputting a result indicating a sum of a
content of two equilibrium species within the aqueous system using
both the cathodic measurement and the anodic measurement.
2. The method of claim 1, wherein the measuring the cathodic
reduction process and measuring the anodic reduction process are
performed substantially simultaneously with respect to one
measurement sample.
3. The method of claim 1, wherein the measuring the cathodic
reduction process and measuring the anodic reduction process are
performed at separate boron doped diamond working electrodes,
respectively.
4. The method of claim 1, wherein the measuring the cathodic
reduction process and measuring the anodic reduction process are
performed consecutively at a single boron doped diamond working
electrode.
5. The method of claim 1 further comprising: measuring hypochlorous
acid (HOCl) by the cathodic measurement; measuring hypochlorite ion
(OCl.sup.-) by the anodic measurement.
6. The method of claim 1, comprising outputting a result indicating
total free chlorine in chlorinated water, as a combination of
measured hypochlorous acid (HOCl) and hypochlorite ion
(OCl.sup.-).
7. The method of claim 1, further comprising: measuring chlorine
dioxide by the cathodic measurement; measuring chlorite by the
anodic measurement.
8. The method of claim 1, wherein the measuring the cathodic
reduction process and measuring the anodic reduction process are
both performed without buffering to control a measurement pH.
9. The method of claim 1, wherein the measuring the cathodic
reduction process and measuring the anodic reduction process are
both performed without the presence of a reagent.
10. The method of claim 1, wherein the working electrodes are bare
working electrodes which are presented directly to the aqueous
system being measured.
11. The method of claim 1, further comprising calibrating a
potential applied in the measuring the cathodic reduction process
by observing a reversal in a mode of a sigmoid shaped response with
respect to varying test potentials.
12. An electrochemical sensor apparatus, comprising: at least one
working electrode of boron doped diamond; signal processing
circuitry operatively connected to the at least one working
electrode and configured to function as: a measurement unit
configured to: measure a cathodic reduction process to provide a
cathodic measurement using a working electrode of boron doped
diamond; measure an anodic oxidation process to provide an anodic
measurement also using a working electrode of boron doped diamond;
a processing unit configured to output a result indicating a sum of
a content of two equilibrium species within an aqueous system using
both the cathodic measurement and the anodic measurement.
13. The apparatus of claim 12, wherein the measuring unit is
configured to perform the measuring the cathodic reduction process
and measuring the anodic reduction process consecutively both on
the same working electrode.
14. The apparatus of claim 12, wherein the measuring unit is
configured to perform the measuring the cathodic reduction process
and measuring the anodic reduction process at the same time on at
least two respective working electrodes.
15. The apparatus of claim 12, further comprising a housing having
a working surface which presents the one or more working electrodes
in a wall-jet configuration.
Description
[0001] This application claims the benefit of United Kingdom
Application No. 1209359.7, filed 28 May 2012, the entire disclosure
of which is incorporated herein by reference.
BACKGROUND
[0002] It is well known to use chlorine as a water additive. For
example, chlorine is applied for disinfection of swimming pools,
for treating drinking water, or during food processing. Hence,
there is a general need for a chlorine analyzer to measure the
presence of chlorine in an aqueous solution. Such chlorine
analyzers are widely needed for measurement in environmental or
industrial situations.
[0003] Known measurement techniques to monitor chlorine in water
on-line are usually based on a wet chemical reagent and optical
measurement, or an electrochemical probe. US2005/029103 (Feng, et
al.) describes an example chlorine sensor of the related art which
measures a chlorine species by electrochemical analysis.
[0004] The known chlorine analyzers are strongly sensitive to the
pH level of the solution being measured. Therefore, typically, a
separate measure of the pH level must be taken in order to
calibrate the measurements of the chlorine analyzer. It would be
desirable to avoid this need for a second sensor to measure pH.
Also, the typical chlorine analyzer is constructed to include a
buffer (e.g., a solution or gel) that stabilizes pH of the water
sample within a measurement chamber. However, it has been noted
that the buffer introduces several disadvantages, such as
complication of the instrument and delay in achieving a
measurement, and thus it would be desirable to avoid the need for a
buffer.
[0005] Generally, it is desired to address one or more of the
disadvantages associated with the related art, whether those
disadvantages are specifically discussed herein or will be
otherwise appreciated by the skilled person from reading the
following description. In particular, it is desired to provide an
electrochemical sensor apparatus and an electrochemical sensing
method which is simple, reliable and cost-effective.
SUMMARY
[0006] According to the present invention there is provided an
electrochemical sensor apparatus and electrochemical sensing method
as set forth in the appended claims. Other features of the
invention will be apparent from the dependent claims, and the
description which follows.
[0007] According to an aspect of the present invention there is
provided an electrochemical sensing method suitable for measuring
an aqueous system. The method includes measuring a cathodic
reduction process using a working electrode of boron doped diamond
(BDD) to provide a cathodic measurement, measuring an anodic
oxidation process using a BDD working electrode to provide an
anodic measurement, and outputting a result indicating a sum of a
content of two equilibrium species within the aqueous system using
both the cathodic measurement and the anodic measurement.
[0008] According to an aspect of the present invention there is
provided an electrochemical sensor apparatus. The apparatus
includes at least one working electrode of boron doped diamond
(BDD). A measurement unit is arranged to measure a cathodic
reduction process to provide a cathodic measurement using a working
electrode of boron doped diamond (BDD), and to measure an anodic
oxidation process to provide an anodic measurement also using a BDD
working electrode. A processing unit is arranged to output a result
indicating a sum of a content of two equilibrium species within the
aqueous system using both the cathodic measurement and the anodic
measurement.
[0009] As will be discussed in more detail below, the example
embodiments address many of the difficulties of the related art. At
least some examples provide a simple, reliable and effective
mechanism for measuring chlorine species in aqueous solutions.
[0010] In one example, the anodic and cathodic measurements may be
performed consecutively at a single BDD working electrode. In
another example, the anodic and cathodic measurements may be
performed at two or more separate working electrodes, respectively.
Surprisingly, it has been found that problems associated with the
pH susceptibility of measurements may be overcome by performing
these two related anodic and cathodic measurements substantially
simultaneously. That is, the anodic and cathodic measurements are
suitably performed at the same time, or consecutively within a
relative short space of time, in relation to substantially the same
measurement sample.
[0011] In one example there is provided an electrochemical sensor
apparatus and electrochemical sensing method for measuring a
disinfectant in an aqueous solution.
[0012] In one example there is provided an electrochemical sensor
apparatus and electrochemical sensing method for measuring chlorine
as a disinfectant.
[0013] In one example, the method and apparatus may be arranged to
measure at least one chlorine atom present in aqueous solutions for
their disinfectant properties. Suitable examples of molecules
comprising at least one chlorine atom include hypochlorous acid,
the hypochlorite ion, chlorine dioxide and the chlorite ion.
[0014] In one example, there is significant interest in measuring
the total free chlorine in chlorinated water, as the combination of
hypochlorous acid (HOCl) and the hypochlorite ion (OCl.sup.-).
Suitably, HOCl is measured by the cathodic measurement, while
substantially simultaneously also measuring OCl.sup.- by the anodic
measurement. Being two equilibrium species, the total free chlorine
is the sum of HOCl and OCl.sup.-. The relative proportions of these
species varies significantly by the measurement pH, while the
[HOCl]/[OCl.sup.-] ratio is constant for any particular pH. Thus,
in the example embodiments, summing the measured concentrations of
HOCl and OCl.sup.- provides the total free chlorine. Notably, the
mechanism is independent of measurement pH.
[0015] In another example, chlorine dioxide and chlorite are
measured by the anodic and cathodic measurements. In this case,
chlorine dioxide is measured by the cathodic (reduction) process,
and chlorite is measured by the anodic (oxidation) process.
[0016] In one example, buffering to control the measurement pH is
not required. Instead, the measurements may be performed at any
suitable pH. The measurements may be performed over a wide range
within the ultimate pH limits of either the reduction and/or
oxidation processes occurring in the anodic and cathodic
measurements.
[0017] In one example, the method may be performed without the
presence of a reagent. Typically, a reagent such as perchlorate
would be required. Although the perchlorate ion seems to enhance
the peak shape of the anodic response to the OCl-- species,
surprisingly it has now been found that it is unnecessary to
include perchlorate in order to obtain a quantitative response.
[0018] In one example, the working electrodes are bare working
electrodes. The working electrodes may be presented directly to the
aqueous system being measured. For example, a wall jet
configuration of the sensor apparatus is now possible. A
measurement chamber or porous membrane now are not required,
leading to a significantly simpler apparatus in some
embodiments.
[0019] These and other features and advantages will be appreciated
further from the following example embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of an example chlorine sensor
apparatus.
[0021] FIG. 2 is a sectional plan view of the chlorine sensor.
[0022] FIG. 3 is a flowchart as a schematic overview of an example
method of measuring chlorine.
[0023] FIG. 4 is a graph of speciation of chlorine in water as a
function of pH.
[0024] FIG. 5 is a graph of a cyclic voltammetric scan of a gold
working electrode as a comparative example.
[0025] FIG. 6 is a graph showing the cathodic and anodic response
of a BDD working electrode towards free chlorine.
[0026] FIG. 7 is a graph showing the cathodic response at a BDD
working electrode in more detail.
[0027] FIG. 8 is a graph showing the anodic response at a BDD
working electrode in more detail.
[0028] FIG. 9 is a graph showing the cathodic response of a
platinum working electrode towards dissolved oxygen.
[0029] FIG. 10 is a graph showing the cathodic response of a BDD
working electrode towards dissolved oxygen.
[0030] FIG. 11 shows the cathodic response of a gold working
electrode towards dissolved oxygen as a comparative example.
[0031] FIG. 12 is a graph illustrating measurement of chlorite and
chlorine dioxide.
[0032] FIGS. 13A-13C are a series of graphs showing calibration
data for anodic response of the BDD working electrode to dissolved
chlorine at different selected potentials.
DETAILED DESCRIPTION
[0033] The example embodiments will be described with reference to
a chlorine sensor apparatus and method, particularly to measure
total free chlorine. The example embodiments described below relate
to the measurement of HOCl and OCl--. In another example, chlorite
and chlorine dioxide may be measured. The apparatus and method may
be applied in many specific implementations, as will be apparent to
persons skilled in the art from the teachings herein.
[0034] FIG. 1 is a perspective view of an example chlorine sensor
apparatus 1. In this example, the sensor apparatus 1 comprises a
main body or housing 10 having one or more working electrodes 11,12
at a working surface thereof. A counter electrode 13 may be
provided. A reference electrode R may also be provided. Optionally
further electrodes may be provided.
[0035] In this example, the housing 10 is generally cylindrical and
the working surface 14 is provided at one end face of the cylinder.
The chlorine sensor is arranged to perform electrochemical
analysis. Conveniently, the sensor obtains and processes
measurements using the working electrodes 11,12 and outputs a
result or data signal by an appropriate communication path. In this
example, the sensor housing 10 is provided with a wired output
connection 15 which allows the sensor to be connected or coupled as
part of a measurement and control system. Other physical
configurations are also envisaged as will be familiar to those
skilled in the art. For example, in a wall jet configuration, it
would be appropriate to place a single working electrode at or
about the geometric center of the generally circular working
surface. It is also envisaged to use concentric working ring
electrodes, with a central disc electrode as a "ring-disc"
configuration within a wall jet flow geometry.
[0036] FIG. 2 is a sectional view of the example sensor 1 through
the sensor body 10. In this example, the counter electrode or
auxiliary electrode 13 is provided as an annular ring at the
working surface 14 surrounding the working electrodes 11,12. This
example apparatus has bare working electrodes 11,12 which are
directly exposed to a flow of water to be sampled. In this example
the sensor is provided in a `wall jet` configuration. A flow of
water W approaches substantially perpendicular to the measuring
surface 14 and is disbursed across the measuring surface to
encounter, inter alia, the working electrodes 11,12 and the
auxiliary electrode 13. Notably, in this example, it is not
necessary to provide the working electrodes 11,12 within a separate
chamber or provide a porous membrane which separates the electrodes
from the main flow of the sample W.
[0037] As shown in FIG. 2, the sensor housing 10 suitably includes
a signal processing unit 20 which is electrically coupled to the
electrodes 11,12,13, etc. A measuring unit 21 contains circuitry
which performs electrochemical analysis using these electrodes. An
output unit 22 prepares a data signal 23 to be output from the
sensor apparatus, such as via the wire 15. It will be appreciated
that many other specific configurations of the apparatus are also
possible. For example, the signal processing unit 20, the measuring
unit 21 and/or the output unit 22 may be provided remote from the
main sensor housing 10, the number, the physical configuration of
the electrodes 11,12,13 may be changed, and so on.
[0038] In one example embodiment, only one working electrode 11 is
required, leading to a simpler and smaller configuration of the
device. In another example, two separate working electrodes 11,12
are provided, which may allow improved measurements. Suitably,
these working electrodes comprise boron doped diamond (BDD). Doped
diamond has been developed as a versatile electrode material and
has been studied in some detail over the past years. However,
several additional interesting and surprising advantages for BDD
electrodes have now been identified, particularly in the context of
chlorine measurement.
[0039] FIG. 3 is a flowchart as a schematic overview of an example
method of measuring chlorine.
[0040] Step 301 comprises measuring an anodic oxidation process to
provide an anodic measurement. This step is performed using any
first one of the one or more working electrodes 11,12.
[0041] Step 302 comprises measuring a cathodic reduction process to
provide a cathodic measurement. Step 302 may be performed again by
the first electrode 11 consecutively before or after the step 301.
Alternately, the step 302 may be performed by a separate second
working electrode 12. Conveniently, the steps 301 and 302 are
performed in close temporal proximity, e.g., at the same time or
within a few seconds of each other, so as to capture measurements
in relation to substantially the same sample.
[0042] Step 303 comprises outputting a result indicating a sum of a
content of two equilibrium species within the aqueous system using
both the cathodic measurement and the anodic measurement.
[0043] It will be appreciated that the anodic and cathodic
measurements of steps 301 and 302 occur when a relevant potential
difference is applied to induce a current flow through the working
electrode. In a typical configuration of the sensor, the counter
electrode 13 is biased relative to the relevant working electrode
11, or vice versa, while the other is held at or near ground
potential. In voltammetry, and particularly in an amperometric
system, the current is measured as a function of time and is
indicative of the concentration of the species being measured.
[0044] As will be familiar to those skilled in the art, chlorine
dissolves in water and establishes the equilibria described by
equations 1 and 2 below:
Cl.sub.2+H.sub.2OHCl+HOCl (Hypochlorous acid) (eqn. 1)
HOClH.sup.++OCl.sup.- (Hypochlorite ion) (eqn. 2)
[0045] Two key species that are present in chlorinated water are
hypochlorous acid and the hypochlorite ion. The relative
proportions of chlorine and these species is controlled principally
by the pH of the water. This is illustrated in FIG. 4, which shows
how these proportions are distributed over the range pH 0 to pH 12.
See also "Residual Chlorine--A guide to measurement in water
applications", Stephen Russell, WRc Instrument Handbooks, WRc plc,
Swindon, FIG. 2, Page 4, 1994. (ISBN 1 898920 17 6).
[0046] The usual range of pH associated with potable water is such
that the principal species present in solution are hypochlorous
acid and hypochlorite ion. It should be noted that at about pH 5,
the speciation is uniquely hypochlorous acid alone, and that above
circa pH 9, the hypochlorite ion predominates.
[0047] The crossing point of the HOCl and OCl.sup.- curves occurs
at pH 7.54 at 25.degree. C. This pH dependency of chlorine
speciation is influential, both in terms of the optimization of
disinfection, and when consideration is given to the measurement of
dissolved chlorine as a process monitoring variable. Hypochlorous
acid has been recognized to be the most effective disinfection
agent of the dissolved chlorine species.
[0048] The chemical speciation in chlorine disinfected water
becomes more complicated when there is a coincident source of
ammonia and related nitrogen compounds. This leads to the formation
of chloramines, through the following sequential reactions:
[0049] The chemical speciation in chlorine disinfected water
becomes more complicated when there is a coincident source of
ammonia and related nitrogen compounds. This leads to the formation
of chloramines, through the following sequential reactions:
HOCl+NH.sub.3.fwdarw.NH.sub.2Cl+H.sub.2O (monochloroamine) (eqn.
3)
NH.sub.2Cl+HOCl.fwdarw.NHCl.sub.2+H.sub.2O (dichloamine) (eqn.
4)
NHCl.sub.2+HOCl.fwdarw.NCl.sub.3+H.sub.2O (trichloramine) (eqn.
5)
[0050] These three reactions are a significant simplification of
the likely reality in chlorinated potable water. The presence of
organic nitrogen sources, such as proteins (which break down to
yield amino acids), further complicate the chemistry of the
chloramines. Hence, measuring chlorine in water is not
straightforward. In the related art, Free Chlorine is typically
used to describe the sum of the concentrations of the inorganic
chlorine species in the water (HOCl and OCl.sup.-). Combined
Chlorine includes the sum of the concentrations of the nitrogenous
chlorine species in the water (chloramines), and Total Chlorine is
usually taken as the sum of the free chlorine and combined chlorine
species.
[0051] Within a sensing system based on reductive amperometry there
is the possibility of interference due to the presence of dissolved
oxygen within the sensing solution, or in a supporting
electrolyte/buffer if used. Dissolved oxygen is known to follow a
two-step reduction process at the cathode, which will be observed
as two distinct voltammetric reduction waves. A first step of the
general type O+n.sub.1e.fwdarw.R.sub.1 is a two electron reduction,
where the H.sub.2O.sub.2 generated is the reduction product,
R.sub.1:
O.sub.2+2H.sub.2O+2e.fwdarw.H.sub.2O.sub.2+2OH.sup.- (eqn. 6)
[0052] A second step of the general type R1+n2e.fwdarw.R2 usually
occurs at significantly more cathodic (negative) potentials:
H.sub.2O.sub.2+2e.fwdarw.2OH.sup.- (eqn. 7)
[0053] Hence, there is a desire to reduce this interference by
dissolved oxygen.
[0054] Conventional free chlorine measurement probes evaluate the
HOCl concentration by electrochemical reduction (at a cathode
working electrode) via the following reaction:
HOCl+2e-.THETA.Cl--+OH.sup.- (eqn. 8)
[0055] The OCl-- species cannot undergo reduction, so does not
register at the cathode working electrode. The current which is
measured at the cathode working electrode is due to the flux of the
electrons supplied from the electrode to promote the reaction in
equation 8. The electron flux, and hence the measured current, is a
function mainly of HOCl concentration and electrode area. Since the
electrode area is fixed, the current should be proportional to HOCl
concentration at the surface of the cathode working electrode. The
concentration of HOCl is also a function of solution pH, according
to the following equation where species concentration is
represented by [HOCl] and [OCl.sup.-] respectively:
log [HOCl]/[OCl.sup.-]=pK.sub.a-pH (eqn. 9)
[0056] The acid dissociation constant, pK.sub.a, as a function of
temperature, T (K) is found by the approximation:
pK.sub.a=3000/T-10.0686+0.0253T (eqn. 10)
[0057] This adds complexity to the typical measurement process,
since a change in the measurement solution pH will result in a
change in the ratio of HOCl species concentration to OCl.sup.-
species concentration. As the pH increases, the concentration of
free HOCl in solution decreases, and the concentration of free
OCl.sup.- in solution increases. The usual way to remove the
experimental variable of pH dependency is to control the pH at the
cathode working electrode by immersing it in a pH buffer (a
chemical reagent that fixes the pH at a pre-determined level). From
the speciation plot in FIG. 4, a pH 5 buffer would tend to maximize
the free solution concentration of HOCl and minimize the
concentration of OCl.sup.-.
[0058] As noted above, the example embodiments employ a dual
measurement mechanism using BDD working electrodes to identify the
respective species independently of pH, in particular to overcome
the pH susceptibility of cathodic amperometric free chlorine
measurements. The dual measurements are characterized by the
substantially simultaneous measurement of both a cathodic
(reduction) and an anodic (oxidation) process. In this example of
free chlorine measurement, the cathodic reaction already described
and as used in conventional free chlorine measurement probes, will
be used in conjunction with the anodic reaction that may be used to
monitor the OCl.sup.- species. The reaction involved is described
by:
6ClO.sup.-+3H.sub.2O.fwdarw.2ClO.sub.3.sup.-+4Cl.sup.-+6H.sup.++3/2O.sub-
.2+6e.sup.- (eqn. 11)
[0059] The simultaneous quantitative measurement of both HOCl and
OCl-- at the same time allows the determination of free chlorine at
any pH, since the free chlorine will be the sum of the
concentrations of HOCl and OCl.sup.-. Thus, the measurement could
be buffered to control the measurement pH, but could equally well
measure at any pH (within the ultimate pH limits of either the
reduction and/or oxidation processes).
[0060] This simultaneous measurement of the two species (HOCl and
OCl.sup.-) might be achieved using a range of electrode materials
(traditionally, platinum, gold, or carbon and, particularly, glassy
carbon). However, a potential limitation with these traditional
electrode materials is their potential range. At the extremes of
their cathodic range, protons in the solution will lead to a
background current, according to the reaction:
2H.sup.++2e.sup.-.fwdarw.H.sub.2 (eqn. 12)
[0061] At the extremes of their anodic range, hydroxyl ions in the
solution will lead to a background current, according to the
two-stage reaction:
2OH.sup.--2e.sup.-.fwdarw.H.sub.2O.sub.2 (eqn. 13)
Then,
H.sub.2O.sub.2-2e.sup.-.fwdarw.2H.sup.++O.sub.2 (eqn. 14)
[0062] Unfortunately, the reality is more complex, since noble
metal electrodes are prone to oxide layer formation at high anodic
potentials. This may be illustrated in FIG. 5 for a gold working
electrode. FIG. 5 is a cyclic voltammetric scan of a gold working
electrode (rotating disc electrode, at 2000 rpm), in a pH 6
phosphate buffer solution, as a comparative example.
[0063] As shown in FIG. 5, the anodic current rises significantly
at an anodic potential more positive than about +0.8 V (vs.
reference electrode), as characterized by the current "hump". The
negative peak at +0.55 V (vs. reference electrode), is the
reduction of the oxide surface back to gold as the potential is
scanned in the cathodic direction. Clearly, such oxide film
formation renders a noble metal electrode unsuitable for operation
at any anodic potential more positive than the potential associated
with the onset of surface oxidation. The negative (cathodic)
current at potentials more negative than +0.1 V (vs. reference
electrode) in this example is due to the reduction of dissolved
oxygen in the solution, according to the reaction given above.
Similar characteristics may be observed for platinum electrodes,
and glassy carbon electrodes are noted for their lack of
reproducibility and gradual passivation when operated at high
anodic potentials. It is, therefore, difficult to utilize
traditional electrode materials for sustained measurement
experiments at high anodic potentials, such as would be required
for the oxidation of the species OCl.sup.-.
[0064] Meanwhile, a simultaneous quantitative measurement of both
HOCl and OCl.sup.- can actually be achieved by using boron doped
diamond (BDD) as the working electrode. BDD has an extremely low
native background current over a very wide potential window in both
cathodic and anodic directions.
[0065] It has been considered to monitor the species OCl.sup.-
through anodic measurement at a BDD working electrode, but previous
examples have consistently employed a highly oxidizing supporting
electrolyte that contains the perchlorate ion. By contrast,
although the presence of the perchlorate ion seems to enhance the
peak shape of the anodic response to the OCl.sup.- species,
surprisingly it has now been found to be unnecessary to include
perchlorate in order to obtain a quantitative response.
[0066] FIG. 6 shows the cathodic and anodic response of a BDD
working electrode towards free chlorine. FIG. 6 also shows typical
applied potentials that could be employed to make cathodic (EC) and
anodic (EA) amperometric measurements. FIG. 6 summarizes the
approximate response of a BDD electrode to successive additions of
free chlorine (thin solid lines), against its background current
(dash-dotted line). The plot represents the response at pH 7.54
(i.e., the pH that corresponds to the pKa of HOCl, where HOCl and
OCl.sup.- species are in 1:1 equilibrium). As pH increases from
7.54, the HOCl (cathodic) response will diminish and the OCl.sup.-
(anodic) response will increase. The converse is true as the pH is
reduced from 7.54. Indeed, the relative responses will conform to
the species equilibrium described above. Thus, the arithmetic sum
of the cathodic response with the anodic response will indicate the
total free chlorine in the solution.
[0067] FIG. 7 shows the cathodic response at a BDD working
electrode to sample solutions loaded at specific concentrations of
free chlorine. In this particular experiment, FIG. 7 shows the
cathodic response of a BDD working electrode towards free chlorine
with supporting electrolyte: 0.05M phosphate buffer at pH6;
electrode rotated at 1000 rpm; linear sweep at 0.05 Vs.sup.-1.
[0068] FIG. 8 shows the anodic response at a BDD working electrode
to sample solutions loaded at a specific concentration of free
chlorine, with varied pH and anodic potential at which the current
has been measured. In this experiment, FIG. 8 shows the anodic
response of a BDD working electrode towards free chlorine, over a
range of pH and at different anodic potentials. The electrode was
rotated at 1000 rpm; linear sweep at 0.05 Vs.sup.-1.
[0069] Generally, the measuring steps may be performed by a sweep
or scan across a voltage range. Measurement samples may be taken
periodically during the sweep or scan. The sweep or scan may be
linear, or may be cyclical. For some species it may be appropriate
to firstly scan or sweep to determine the presence of peaks (which
may vary for example based on PH or temperature) and then determine
the most appropriate measurement points within the scan or
sweep.
[0070] These experimental examples have demonstrated the link
between cathodic measurement of the HOCl species and the anodic
measurement of the OCl.sup.- species. It also seems that BDD is
less prone to interference from the presence of dissolved oxygen in
the sample. This is less important for a membrane mediated
amperometric probe, since a steady state will be achieved such that
any background current due to dissolved oxygen will be constant and
small. However, this would not be the case for membraneless
systems, where sudden fluctuations in dissolved oxygen will affect
the measurement current of the probe system. Notably, a bare
electrode chlorine sensor is now feasible.
[0071] FIG. 9 shows the cathodic response of a platinum working
electrode towards dissolved oxygen. For comparison, the effect of
dissolved oxygen on the background response of a platinum working
electrode is shown. The scan numbers are at fixed intervals with
exposure of the sample buffer to laboratory air. Scan 01 is the
background after dissolved air/oxygen had been expelled from the
sample by sparging with helium. Here, the initial measurement (scan
01) is in air/oxygen free buffer, and is therefore the background
current for the platinum electrode in the pH 6 phosphate buffer.
Subsequent scans are monitored as the solution is progressively
exposed to laboratory air. Scan 40 represents the steady state
response to the buffer after it has reached equilibration with the
laboratory air. Subsequent scans would appear superimposed on the
Scan 40 plot.
[0072] FIG. 10 shows the cathodic response of a BDD working
electrode towards dissolved oxygen. FIG. 10 is a plot of a degassed
and an air saturated buffer solution (0.5M lithium ethanoate, pH5).
It is clear from these data that not only is the background less
affected by the dissolved oxygen, but also that the background
current is substantially less. (Compare the current scales:
Platinum 0 to -140 .mu.A; BDD 0 to -1.8 .mu.A).
[0073] FIG. 11 shows the cathodic response of a gold working
electrode towards dissolved oxygen as a comparative example. For
the sake of completeness, a similar plot is shown in FIG. 11 for a
gold working electrode with the same electrolyte as used in FIG.
10, as a direct comparison between gold and BDD. The difference in
current scales should again be noted. (It should be noted that the
peaks at +1.0V (anodic) and +0.6V (cathodic) are the oxidation of
the gold surface and the reduction of gold oxide respectively).
[0074] The principle has been illustrated and exemplified with
reference to free chlorine measurement where there are two distinct
species that make up an equilibrium composition that is pH
dependent. The purpose of making two measurements is to overcome pH
sensitivity that is inherent in the speciation chemistry of any
sample under observation, where deliberate fixing of the pH through
buffering is either undesirable, infeasible, or has only partial
effectiveness.
[0075] Other possible assays include similar equilibrium coupling
of species that occur and are governed by pH. Also, the
simultaneous measurement of systems that are self-reversible may be
candidates for this approach. An example of this that is of
significance to water quality monitoring are the species chlorine
dioxide and chlorite, which are related through the following
reaction:
ClO.sub.2+e.sup.-.fwdarw.ClOC2.sup.- (eqn. 15)
[0076] A BDD working electrode may be used to measure chlorine
dioxide through its cathodic reduction to the chlorite ion, and
also used to measure the chlorite ion through its anodic oxidation
to chlorine dioxide. Thus, a single electrode may be used to
monitor both species, simply through the control of the applied
potential. Similarly to the free chlorine measurement, both
chlorine dioxide and chlorite ion may be measured simultaneously by
using a combination of a cathodic and anodic assay.
[0077] FIG. 12 is a graph showing data for chlorite anodic
oxidation (topside curves) at ca. +1.0V. This process of chlorite
oxidation generates chlorine dioxide, which accumulates at a
stationary (no flow, no stirring) BDD working electrode. The
reduction of the accumulated chlorine dioxide is clearly visible on
the cathodic measurement (underside curves) at ca. +0.4V. Note the
response is less for the chlorine dioxide, since the bulk solution
contains the chlorite, but it is only the chlorine dioxide that
remains near the electrode surface that can be measured in this
experiment. The concentrations are the bulk values for
chlorite.
[0078] At the cathode (reduction-addition of electron), chlorine
dioxide is reduced to chlorite as shown in Equation 15 above. At
the anode (oxidation-removal of electron), chlorite is oxidized to
chlorine dioxide:
ClO.sub.2.sup.-.fwdarw.ClO.sub.2+e (eqn. 16)
[0079] FIGS. 13A-13C are a series of graphs showing calibration
data for anodic response of the BDD working electrode at different
selected potentials. In a further enhancement, it has been found
that the anodic response of the BDD electrodes exhibits observable
nonlinearity compared with an ideal linear regression. The response
curve as illustrated in FIG. 13A and FIG. 13B is typically a
sigmoid. Interestingly, the sigmoid deviates around an ideal linear
response and the direction of deviation reflects to an opposing
direction as the voltage is varied. It has been found that the
sensor apparatus may be calibrated by adjusting the applied
potential to produce a substantially linear response at or about
the point where this deviation inverts. When the potential is lower
than ideal, as in FIG. 13A, then the sigmoid deviates in one mode,
and when too high, as in FIG. 13B, deviation is observed in an
opposing mode. Between these ranges lies a potential which produces
a more or less linear response, as is illustrated in FIG. 13C.
Thus, the method suitably includes the step of calibrating the
anodic potential EA by observing a reverse in the mode of the
sigmoid response curve. In the example embodiments, the measuring
unit 21 may perform such a corresponding calibration function.
[0080] The industrial application of the present invention will be
clear from the discussion above. The advantages of the invention
have also been discussed and include providing a simple, reliable
and efficient mechanism for sensing chlorine species. In some
embodiments, a pH buffer or a reagent are not required.
[0081] Although a few preferred embodiments have been shown and
described, it will be appreciated by those skilled in the art that
various changes and modifications might be made without departing
from the scope of the invention, as defined in the appended
claims.
[0082] The present invention may, of course, be carried out in
other ways than those specifically set forth herein without
departing from essential characteristics of the invention. The
present embodiments are to be considered in all respects as
illustrative and not restrictive, and all changes coming within the
meaning and equivalency range of the appended claims are intended
to be embraced therein.
* * * * *